1. Technical Field
The field of this invention concerns segment polarity genes and their uses.
2. Background
Segment polarity genes were discovered in flies as mutations which change the pattern of structures of the body segments. Mutations in the genes cause animals to develop the changed patterns on the surfaces of body segments, the changes affecting the pattern along the head to tail axis. For example, mutations in the gene patched cause each body segment to develop without the normal structures in the center of each segment. In their stead is a mirror image of the pattern normally found in the anterior segment. Thus cells in the center of the segment make the wrong structures, and point them in the wrong direction with reference to the over all head-to-tail polarity of the animal. About sixteen genes in the class are known. The encoded proteins include kinases, transcription factors, a cell junction protein, two secreted proteins called wingless (WG) and hedgehog (HH), a single transmembrane protein called patched (PTC), and some novel proteins not related to any known protein. All of these proteins are believed to work together in signaling pathways that inform cells about their neighbors in order to set cell fates and polarities.
Many of the segment polarity proteins of Drosophila and other invertebrates are closely related to vertebrate proteins, implying that the molecular mechanisms involved are ancient. Among the vertebrate proteins related to the fly genes are En-1 and -2, which act in vertebrate brain development and WNT-1, which is also involved in brain development, but was first found as the oncogene implicated in many cases of mouse breast cancer. In flies, the patched gene is transcribed into RNA in a complex and dynamic pattern in embryos, including fine transverse stripes in each body segment primordium. The encoded protein is predicted to contain many transmembrane domains. It has no significant similarity to any other known protein. Other proteins having large numbers of transmembrane domains include a variety of membrane receptors, channels through membranes and transporters through membranes.
The hedgehog (HH) protein of flies has been shown to have at least three vertebrate relatives: Sonic hedgehog (Shh); Indian hedgehog, and Deser hedgehog. The Shh is expressed in a group of cells at the posterior of each developing limb bud. This is exactly the same group of cells found to have an important role in signaling polarity to the developing limb. The signal appears to be graded, with cells close to the posterior source of the signal forming posterior digits and other limb structures and cells farther from the signal source forming more anterior structures. It has been known for many years that transplantation of the signaling cells, a region of the limb bud known as the xe2x80x9czone of polarizing activity (ZPA)xe2x80x9d has dramatic effects on limb patterning. Implanting a second ZPA anterior to the limb bud causes a limb to develop with posterior features replacing the anterior ones (in essence little fingers instead of thumbs). Shh has been found to be the long sought ZPA signal. Cultured cells making Shh protein (SHH), when implanted into the anterior limb bud region, have the same effect as an implanted ZPA. This establishes that Shh is clearly a critical trigger of posterior limb development.
The factor in the ZPA has been thought for some time to be related to another important developmental signal that polarizes the developing spinal cord. The notochord, a rod of mesoderm that runs along the dorsal side of early vertebrate embryos, is a signal source that polarizes the neural tube along the dorsal-ventral axis. The signal causes the part of the neural tube nearest to the notochord to form floor plate, a morphologically distinct part of the neural tube. The floor plate, in turn, sends out signals to the more dorsal parts of the neural tube to further determine cell fates. The ZPA was reported to have the same signaling effect as the notochord when transplanted to be adjacent to the neural tube, suggesting the ZPA makes the same signal as the notochord. In keeping with this view, Shh was found to be produced by notochord cells and floor plate cells. Tests of extra expression of Shh in mice led to the finding of extra expression of floor plate genes in cells which would not normally turn them on. Therefore Shh appears to be a component of the signal from notochord to floor plate and from floor plate to more dorsal parts of the neural tube. Besides limb and neural tubes, vertebrate hedgehog genes are also expressed in many other tissues including, but not limited to the peripheral nervous system, brain, lung, liver, kidney, tooth primordia, genitalia, and hindgut and foregut endoderm.
PTC has been proposed as a receptor for HH protein based on genetic experiments in flies. A model for the relationship is that PTC acts through a largely unknown pathway to inactivate both its own transcription and the transcription of the wingless segment polarity gene. This model proposes that HH protein, secreted from adjacent cells, binds to the PTC receptor, inactivates it, and thereby prevents PTC from turning off its own transcription or that of wingless. A number of experiments have shown coordinate events between PTC and HH.
Descriptions of patched, by itself or its role with hedgehog may be found in Hooper and Scott, Cell 59, 751-765 (1989); Nakano et al., Nature, 341, 508-513 (1989) (both of which also describes the sequence for Drosophila patched) Simcox et al., Development 107, 715-722 (1989); Hidalgo and Ingham, Development, 110, 291-301 (1990); Phillips et al., Development, 110, 105-114 (1990); Sampedro and Guerrero, Nature 353, 187-190 (1991); Ingham et al., Nature 353, 184-187 (1991); and Taylor et al., Mechanisms of Development 42, 89-96 (1993). Discussions of the role of hedgehog include Riddle et al., Cell 75, 1401-1416 (1993); Echelard et al., Cell 75, 1417-1430 (1993); Krauss et al., Cell 75, 1431-1444 (1993); Tabata and Kornberg, Cell 76, 89-102 (1994); Heemskerk and DiNardo, Cell 76, 449-460 (1994); Relink et al., Cell 76, 761-775 (1994); and a short review article by Ingham, Current Biology 4, 347-350 (1994). The sequence for the Drosophila 5xe2x80x2 non-coding region was reported to the GenBank, accession number M28418, referred to in Hooper and Scott (1989), supra. See also, Forbes, et al., Development 1993 Supplement 115-124.
Methods for isolating patched genes, particularly mammalian patched genes, including the mouse and human patched genes, as well as invertebrate patched genes and sequences, are provided. The methods include identification of patched genes from other species, as well as members of the same family of proteins. The subject genes provide methods for producing the patched protein, where the genes and proteins may be used as probes for research, diagnosis, binding of hedgehog protein for its isolation and purification, gene therapy, as well as other utilities.